The present invention relates generally to semiconductor optical devices. More particularly, the invention relates to an optical device having a plurality of semiconductor lasers of different output wavelengths formed on a single semiconductor substrate, notably a semiconductor optical device suitable for frequency or wavelength multiplexing data transmission through optical communication modules, optical communication systems or optical communication networks.
Frequency or wavelength multiplexing schemes are actively considered with a view to expanding the transmission capacity of optical communication systems. In particular, efforts are under way for research and development of an integrated array light source having a plurality of single-mode lasers of a different oscillation wavelength each formed on the same substrate. This light source is an important device for use with the frequency or wavelength multiplexing schemes currently contemplated. Conventional multi-wavelength laser arrays reportedly comprise active layers having the same emission peak wavelength and formed of the same crystal growth for all wavelengths. In such setups, electron beam lithography equipment or the like is employed to control the period of the diffraction grating so as to provide any of multiple wavelengths. A report on such a scheme is found in the Optics and Photonics News (O.P.N.), pp. 24-26, March 1993, the Optical Society of America.
The need to utilize the electron beam lithography equipment or the like complicates the above scheme of implementing multi-wavelength laser arrays. Furthermore, attempts to make semiconductor laser characteristics uniform are adversely affected by the fact that the difference between the oscillation wavelength in question and the gain peak wavelength of the active layer (i.e., amount of detuning) varies from one semiconductor laser to another. Reports on these aspects have indicated that although the oscillation wavelength is controlled, increased amounts of detuning lead to significant drops in laser oscillation threshold values and in oscillation efficiency. Also affected adversely are relaxation frequency and spectral line width, which are important factors in the case of rapid modulation. This is one of the problems getting more serious the greater the number of semiconductor lasers (i.e., channel count) formed on a single semiconductor substrate and the wider the range of frequencies or wavelengths used. The problem must be resolved before frequency or wavelength multiplexing transmission can receive widespread acceptance.
There have been reports on a distributed Bragg reflector laser that effects emissions of different wavelengths by use of the diffraction grating of the same period. The emissions are achieved by controlling the layer thickness of the optical waveguide through its selective growth by metal organic vapor epitaxy. One such report was submitted to the 54th Japanese Applied Physics Conference, 27p-H-18, September 1993. With this type of distributed Bragg reflector laser, the active layers are also made of a material having the same gain peak wavelength. The active layer constitution can also lead to the semiconductor laser characteristics getting uneven.